Just changed mine tonight. It was shorted to ground and was blowing the breaker instantly when plugged in. Bought a new cord to test, and that one would short to ground instantly too. Knew it was the block heater. Heater was leaking right where the electrical prongs are soldered into the heater. You could see the coolant weeping there.

I was ready to remove the passenger side engine mounts while supporting the engine from beneath to get at it (the GM recommended procedure...mostly designed to separate customers from their money), but another guy on a forum said he didn't have to, so I thought I'd try without removing the engine mounts, but ready to do so.

I found that it is not necessary on a NBS (GMT900) duramax LMM to remove the engine mounts to replace the block heater. Note, the GM part is made in CHINA. I didn't want to use it but I had it handy just in case. It is double the cost of the made in Canada "Temro" brand that I wanted to use, which I also had on hand because I'll always favor north american made. Anyhow the plug portion of the heater (that the element is mounted into) comes in 2 configurations. Think of a manhole cover. What stops the cover from dropping through the hole, I'm calling a "flange". That flange goes all the way around on the Temro and on the GM part, the flange consists of 3 "Tabs" (one each at 12:00, 4:00 and 8:00) that prevent the heater from pushing through into the block.

The GM style makes it possible to get the heater past the aluminum heat shield on the engine mounts (the shield is proud of the engine mount by 1/4"). I used a die grinder on the heat shield to remove a 1/4" from the heat shield so I could get the Temro in. However, I found that the factory thermostatically controlled block heater plug wouldn't plug into the temro heater. These trucks are rumored to throw a code if you heat the coolant above freezing temps, making retention of the thermostat plug (and the GM heater) necessary.

Anyhow, that's why I had to use the GM part. Temro would be a better heater (since it's made in Canada rather than China, and it's half the price of the chinese part), and Temro does sell the thermostat plug/wire separately. Anyhow, here's what I did.

1. Jack up the truck and raise the passenger side front wheel off the ground, supporting with jackstands.

2. remove the passenger side front wheel.

3. remove inner fender well.

4. remove plastic fenderwell mount at front of fenderwell by pulling 2 plastic push connectors. (It's kind of below the lower rad hose and would cause a mess because the coolant would drain onto it if not removed)

5. remove lower radiator hose by pulling large spring clip out and pulling hose towards rear of truck. about 14L of coolant came out.

6. using a small metric hex (male) socket (4mm, to be exact), thread out the toggle bolt on the block heater and pry gently until you can break it loose. Pry gently until both retaining "wings" bend back enough to get the unit out. Look at the new block heater to get a feel for how these retainers work. Do not fully remove the bolt, otherwise the wings will drop into the engine block, potentially blocking a coolant passage or getting into your water pump and fragging it.

7. Clean/sand/debur the opening in the block making sure it is perfect to ensure a perfect seal.

8. Apply GM all purpose high temp grease to the O-ring mounted on the new block heater and shimmy it back in. (If using a full-flanged heater, use a die grinder or dremel to remove excess heat shield in the way. If using the triple tab style, don't worry about it).

9. Install block heater and torque to spec (in block heater instructions). The spec is really low, like 18 inch pounds or something very small like that. Don't quote me on this.

10. Plug in block heater to a cord that you know is good.

11. Re-install lower rad hose/clip

12. fill coolant through recovery bottle.

13. if you get air in the system, you can bleed it through the bleed screw located in the top of the thermostat housing. This is under the upper mount for your oil fill hole. It is the only bolt that doesn't hold anything down.

14. consider changing your fuel filter at this time. It's right there. Dang it, I didn't have one on the shelf and it was too late to get one.

Job well done, and saved my self $600 in labor according to my local GM Stealership.

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